1. Field of the Invention
The present invention relates to a focus detecting system and, more particularly, to a focus detecting system which performs focus detection for an optical system on the object, the optical system being so set as to cause changes in the image formation state of the image of the object in accordance with data processing of an image formation state of an image of an object formed on a solid image-pickup element such as a CCD, BBD, or CID or in an image pickup tube.
2. Description of the Prior Art
Various focus detecting systems for optical systems have been conventionally proposed. For example, an increasing number of focus detecting systems have been proposed which utilize solid image-pickup elements such as the CCD, BBD or CID, the practicability of which has recently become notable.
The characteristic feature of the image pickup tube or the solid image-pickup element referred to above resides in that extremely small photoelectric transducer elements are incorporated which output time-serial electric signals of small picture elements of an image formed on the tube. Therefore, the image pickup tube and the solid image-pickup element allow subsequent time-serial processing of the signals and therefore are suitable for processing with electric circuitry, in contrast to a conventional system which has a plurality of usual photoelectric elements on which the image of an object is formed to obtain photoelectric conversion signals of the image. Since the photoelectric transducer elements constituting the solid image-pickup element, unlike the usual photoelectric transducer elements, serve to store for a given period of time charges obtained by photoelectrically converting the energy of light incident thereon and to output time-serial signals, the area for each of these elements may be made very small so that the obtained time-serial signal train provides image signals of good resolution.
On the other hand, with a focus detecting system using an accumulating-type photoelectric transducer element, an abrupt change in the brightness of the object or an abrupt movement of the image on the accumulating-type photoelectric transducer elements results in an abrupt change in the accumulating time. This, in turn, results in an extremely unstable photoelectric output and thus an extremely unstable final output. For this reason, confirmation of the focusing state or control for focusing an optical system on an object may occasionally become impossible to perform.
In addition to this, with a focus detecting system of the type described above, the output signals corresponding to the focusing state are generated at time intervals corresponding to the accumulating time of the elements. Therefore, when such a system is applied to a camera, depending upon the focal length of the photographic lens used or upon the presence or absence of shaking, the image on the photoelectric transducer elements uninterruptedly moves in relatively great magnitudes, resulting in changes in the brightness and pattern of the image. The corresponding changes in the accumulating time and the focusing signals make the signals representing the focusing state unstable. When these signals are applied to an LED for indication of the focusing state, the LED flickers and does not provide a positive indication. When the system is applied to automatic focusing of a photographic lens, control of the photographic lens disadvantageously becomes unstable.